This invention relates to optical fiber communication systems and, more particularly, to an arrangement for tapping optical power from an optical fiber waveguide without requiring that the fiber be terminated or broken.
Rapid progress has been made in the past few years in the design and fabrication of optical fiber wave-guiding structures. There are now available several different fiber structures which are capable of transmitting large quantities of information via modulated optical waves or pulses with transmission losses as low as two decibels per kilometer. It is expected that some day such fibers will replace, at least in part, the wire pairs, coaxial cables and metallic waveguides now used in conventional communication systems. The advantages of fiber systems over conventional systems include the small physical size and light weight of the fiber waveguides, the broad bandwidth capabilities which afford flexibility in the selection of a bandwidth to be utilized in any given system, the nonconductive, noninductive properties of the fiber waveguides, and the potentially low cost of fiber materials and fabrication. The prospects of future use of the fiber systems are indeed wide-ranging, and continue to expand.
One early implementation of fiber systems is likely to involve multi-terminal information transfer over short distance optical fiber links using light-emitting diodes, which have now been developed to the point of having sufficiently long operating lives, as the signal sources. Because of the light weight and the immunity to electromagnetic interference of fiber systems, fiber optical data bus links have been proposed for the transmission of control and intercom signals on board aircraft and ships. Other potential applications include interoffice trunks, such as those interconnecting telephone central offices within a city, "on premise" distribution links within a building or between adjacent buildings, and data bus links in computer or industrial-control systems.
In the longer range future, fiber systems are likely to be used for the high capacity transmission of digital information over long distance fiber links, with lasers as the signal source. Intercity telecommunication links may thus some day be provided using optical fibers. It appears likely that repeater spacings of several kilometers or more and information transmission rates up to the gigabit range will become technically feasible with such systems.
Whatever the application, it is clear that arrangements will be required for extracting signal wave information from the optical fiber waveguides. To monitor and control the transmission through a fiber link, for example, it may be required to sample the signal propagating through the individual fiber waveguides periodically along the link. Optical data bus links will require that signals be extracted for utilization at numerous selected points along the link. In most instances, it would be desirable if a portion of the signal propagating through the fiber could be tapped therefrom without breaking or terminating the fiber. Fiber terminations can add unwanted optical losses to the system, and would unfavorably increase the need for highly precise fiber splicing and interconnecting arrangements.
In the concurrently filed applications of J. E. Goell, T. Li and W. M. Muska, Ser. No. 522,577, and of W. M. Muska, Ser. No. 522,518, there is disclosed a variety of arrangements for tapping signal power from an intermediate portion of an optical fiber waveguide without requiring that the fiber be terminated or broken. In each illustrative embodiment of the optical fiber power tap disclosed in the cited applications, power is coupled out of the fiber waveguide by a dielectric body disposed in a coupling relationship with an intermediate length of the fiber, and is converted to a representative electrical signal suitable for utilization by a photodetector disposed adjacent to the dielectric body. To tap clad fiber waveguides, all or most of the outer cladding is removed from the fiber in the vicinity of the fiber tap so that the dielectric body of the tap can extract power directly from the inner core. The dielectric body of the tap is disposed at least within about three optical wavelengths of the inner core to achieve the desired coupling relationship. Alternatively, the fiber is bent in the vicinity of the fiber tap to cause a portion of the optical power to radiate out of the inner core into the outer cladding from which it can be extracted by the dielectric body. In either case, some fraction of the power is tapped from the fiber, provided the index of refraction of the dielectric body is approximately equal to, or greater than, the index of refraction of the outer cladding of the fiber.
With multimode fiber waveguides, that is, with fiber waveguides in which the optical power is distributed among a plurality of propagating modes of different orders, the dielectric coupling body of the fiber tap tends to extract only the higher order mode power from the fiber at the point where it is attached. Accordingly, when one or more fiber taps are added further along the fiber, there is typically less power in the higher order modes for the additional taps to couple out. Systems requiring multiple taps closely spaced along a single fiber could thus be difficult to implement.